Support vsync-aligned wheel event dispatch

content/browser/renderer_host/input/wheel_event_queue.cc

Index: content/browser/renderer_host/input/wheel_event_queue.cc
diff --git a/content/browser/renderer_host/input/wheel_event_queue.cc b/content/browser/renderer_host/input/wheel_event_queue.cc
new file mode 100644
index 0000000000000000000000000000000000000000..2d3131e7c053979ca4c041d6d0cbc0a26ac06c51
--- /dev/null
+++ b/content/browser/renderer_host/input/wheel_event_queue.cc
@@ -0,0 +1,111 @@
+
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "content/browser/renderer_host/input/wheel_event_queue.h"
+
+#include "base/auto_reset.h"
+#include "base/metrics/histogram.h"
+
+namespace content {
+
+WheelEventQueue::WheelEventQueue(WheelEventQueueClient* client,
+                                 bool buffer_until_flush)
+    : client_(client),
+      buffer_until_flush_(buffer_until_flush),
+      mouse_wheel_pending_(false),
+      flushing_(false) {}
+
+WheelEventQueue::~WheelEventQueue() {}
+
+void WheelEventQueue::QueueEvent(
+    const MouseWheelEventWithLatencyInfo& wheel_event) {
+  if (mouse_wheel_pending_ || buffer_until_flush_) {
+    // If there's already a mouse wheel event waiting to be sent to the
+    // renderer, add the new deltas to that event. Not doing so (e.g., by
+    // dropping the old event, as for mouse moves) results in very slow
+    // scrolling on the Mac.
+    DCHECK_IMPLIES(wheel_event.event.hasPreciseScrollingDeltas,
+                   wheel_event.event.canScroll);
+    if (!coalesced_wheel_events_.empty() &&
+        coalesced_wheel_events_.back().CanCoalesceWith(wheel_event)) {
+      coalesced_wheel_events_.back().CoalesceWith(wheel_event);
+      TRACE_EVENT_INSTANT2("input", "InputRouterImpl::CoalescedWheelEvent",
+                           TRACE_EVENT_SCOPE_THREAD, "total_dx",
+                           coalesced_wheel_events_.back().event.deltaX,
+                           "total_dy",
+                           coalesced_wheel_events_.back().event.deltaY);
+    } else {
+      coalesced_wheel_events_.push_back(wheel_event);
+    }
+    if (buffer_until_flush_)
+      client_->SetNeedsFlush();
+    return;
+  }
+
+  SendEvent(wheel_event);
+}
+
+void WheelEventQueue::ProcessWheelAck(InputEventAckState ack_result,
+                                      const ui::LatencyInfo& latency) {
+  DCHECK(mouse_wheel_pending_);
+  // TODO(miletus): Add renderer side latency to each uncoalesced mouse
+  // wheel event and add terminal component to each of them.
+  current_wheel_event_.latency.AddNewLatencyFrom(latency);
+
+  // Process the unhandled wheel event here before calling |SendEvent()|
+  // since it will mutate current_wheel_event_.
+  client_->OnWheelEventAck(current_wheel_event_, ack_result);
+
+  // Mark the wheel event complete only after the ACKs have been handled above.
+  // ACKing the event could cause another event to be enqueued, and that
+  // event's sending should be deferred.
+  mouse_wheel_pending_ = false;
+
+  // Send the next (coalesced or synthetic) mouse wheel event.
+  if (coalesced_wheel_events_.empty())
+    return;
+
+  if (buffer_until_flush_ && !flushing_) {
+    client_->SetNeedsFlush();
+    return;
+  }
+
+  MouseWheelEventWithLatencyInfo next_wheel_event =
+      coalesced_wheel_events_.front();
+  coalesced_wheel_events_.pop_front();
+  SendEvent(next_wheel_event);
+}
+
+void WheelEventQueue::Flush() {
+  if (!buffer_until_flush_)
+    return;
+
+  if (mouse_wheel_pending_)
+    return;
+
+  if (coalesced_wheel_events_.empty())
+    return;
+
+  base::AutoReset<bool> flushing(&flushing_, false);
+  MouseWheelEventWithLatencyInfo next_wheel_event =
+      coalesced_wheel_events_.front();
+  coalesced_wheel_events_.pop_front();
+  SendEvent(next_wheel_event);
+  DCHECK(mouse_wheel_pending_);
+}
+
+void WheelEventQueue::SendEvent(
+    const MouseWheelEventWithLatencyInfo& wheel_event) {
+  DCHECK(!mouse_wheel_pending_);
+  mouse_wheel_pending_ = true;
+  current_wheel_event_ = wheel_event;
+
+  LOCAL_HISTOGRAM_COUNTS_100("Renderer.WheelQueueSize",
+                             coalesced_wheel_events_.size());
+
+  client_->SendWheelEventImmediately(wheel_event);
+}
+
+}  // namespace content